Typically, tuning of a VCO is achieved by changing the capacitance of the LC resonator tank. In order to obtain a wide tuning range, several techniques are used. Metal-oxide-semiconductor (MOS) varactors support a large capacitance variation, and have high tuning sensitivity and low phase noise performance. Switched capacitor banks have resistive and capacitive parasitics associated with the switches. Further, inductor switching has been proposed, although this technique is limited by the use of the switches.
Further, it has been proposed to employ a transformer-based resonator. A transformer-based resonator features two different modes of oscillation and thus provides for a wider frequency range. Switching between two modes of oscillation is accomplished by closing a switch in the second resonator circuit causing the second resonator circuit to be inductively coupled to the first resonator circuit. This changes the overall inductive impedance seen at the first resonator circuit resulting in a shift of the resonance frequency of the first resonator circuit.